1. Field of the Invention
The present invention relates to carbon-carbon composites, and more particularly to composites made from fibrous carbon material, a thermosetting resin, a boron containing compound, and a refractory metal capable of reacting with said boron containing compound to form a metal boride. This invention also relates to fibrous carbon material impregnated with a thermosetting resin binder useful in the preparation of such carbon-carbon composites and to a method for making such composites.
2. Description of the Prior Art
It is known to use boron in the manufacture of carbon material such as graphite made from a filler such as graphite powder and graphitizable material such as pitch or a resin. The boron enhances the combination of the materials and the conversion thereof into graphite.
It is also known in the art to use boron in the manufacture of carbon-carbon composites comprised of fibrous carbon material such as carbon or graphite cloth and a thermosetting resin. Examples of this type of composite are disclosed in U.S. Pat. No. 3,672,936, issued June 27, 1972 to Leo C. Ehrenreich. The Ehrenreich patent recognizes that there is some improvement in interlaminar tensile strength as well as in oxidation resistance when a boron containing compound is added to the resin impregnated fibrous carbon material prior to carbonization of the resin.
U.S. Pat. No. 4,101,354 to Robert C. Shaffer discloses that the interlaminar tensile strength of carbon-carbon composites containing boron is greatly improved if the composite is heated to at least about 2150.degree. C. during carbonization and graphitization and, further, that at such temperatures the tensile strength in the directions of the fibers of the fibrous carbon material typically decreases substantially, apparently due to a deterioration of the fibrous carbon material of the composite caused by reaction with boron at high temperatures. In accordance with the teachings of that patent, significant decrease in the tensile strength in the directions of the fibers of the fibrous carbon material is prevented by use of a protective coating on the fibers. The protective coating comprises a thermosetting material which remains flexible after being subjected to curing temperatures. The coating is applied to the fibers and cured prior to addition of a resin and a boron containing compound. The resin and boron containing compound may then be added with the resin being at least partially cured. Upon formation of a laminate and heating of the laminate to a temperature sufficient to carbonize and at least partially graphitize the resin, the interlaminar tensile strength has been found to be greatly improved without significant decrease in the tensile strength in the directions of the fibers of the fibrous carbon material of the laminate. The protective resin coating on the fibers creates a barrier and results in an anisotropic composite even though high levels of boron are present in the matrix. However, this barrier is insufficient to limit boron migration and to conserve the anisotropic nature of the composite when the high temperature consolidation temperature exceeds 2482.degree. C. Above this temperature, the high levels of boron exhibit such instability that either an isotropic composite results or the composite fails by gross fracture.